Bump-on-Trace (BOT) structures were used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is low. The conventional BOT structures may achieve the same reliability as the conventional bond structures that are based on metal pads.
With the increasing down-scaling of integrated circuits, the metal traces become increasingly narrower. The metal traces in a die are formed over a dielectric layer, and are adhered to the underlying dielectric layer mainly by anchoring forces, which are generated mainly due to the surface roughness of the dielectric layer. Narrow metal traces have small contact areas with the underlying dielectric layer, and hence the anchoring forces are small. Hence, the delamination between the metal traces and the dielectric layer becomes a severe problem.